1. Field of the Invention
The present invention relates to a tire air pressure monitoring system designed to monitor an air pressure state of each of tires of a vehicle.
2. Description of the Related Art
So far, there have been proposed various types of systems for monitoring an air pressure of each of tires of a vehicle. One of such tire air pressure monitoring systems is disclosed in Japanese Patent No. 3212311 in which a sensor unit comprising a sensor for measuring a tire air pressure and a transmitter for transmitting the tire air pressure measurement value is provided in each of tires and a monitoring unit receives a signal transmitted from the sensor unit to address a warning if each tire air pressure measurement value falls below a predetermined value.
In this conventional tire air pressure monitoring system, for grasping the position of the tire equipped with each of the sensor units, a specific ID code allocated to each of the sensor units is registered in the monitoring unit in a state associated with the tire position. With this arrangement, when receiving a transmitted signal including an air pressure measurement value and an ID code, the monitoring unit can grasp, on the basis of the ID code, the sensor unit which has made the signal transmission and the tire holding that sensor unit. Accordingly, since the tire position is detectable in this way, when a decision is made, on the basis of the air pressure measurement value, that the tire air pressure is in an abnormal condition, it is possible to specify the tire position and to make the notification about the abnormality of the tire air pressure.
However, the aforesaid registration of the ID code of each of the sensor units in the monitoring unit in a state associated with the tire position requires troublesome operations. That is, the operating mode of the monitoring unit is switched from the monitoring mode to the registration mode through the switch operation on the monitoring unit and the tire position is set for the registration of the ID code. In this state, for example, by rapidly decreasing the tire air pressure, an ID code is forcibly transmitted from the sensor unit at the corresponding tire position to be registered in the monitoring unit. These operations are conducted with respect to all the tires of a vehicle.
In addition, even in a case in which the ID code of each of the sensor units is registered in the monitoring unit in a state associated with the tire position through the aforesaid operations, if the vehicle' user carries out the tire rotation in order to uniformize the abrasive states of the tires, the association between the registered ID codes and the tire positions makes no sense anymore.